1. Field of the Invention
Aspects of the present invention relate to an apparatus and a method of outputting stereophonic sound, and more particularly, to an apparatus and a method of outputting stereophonic sound in which a 5.1 channel audio signal is down-mixed to a 2-channel audio signal to be output to headphones.
2. Related Art
As digital stereophonic systems, such as digital broadcasting and digital video disc (DVD) players, have become widely used, 5.1 channel sound also is being commonly utilized. The 5.1 channel sound may be played back through a sound system that is arranged according to a user's needs, and provides three-dimensional stereophonic sound to the user. Since output devices of sound systems, such as computers or portable sound apparatuses, can output 2-channel sounds through two speakers, the 5.1 channel audio signal is down-mixed in these systems to a 2-channel audio signal using a predetermined signal process in order to enjoy the 5.1 channel sound.
FIGS. 1A and 1B are diagrams explaining a conventional method of outputting a stereophonic sound. In FIG. 1A, speakers 2, 3, 4, 5, and 6 are arranged around a center where a user 1 is located. A sub woofer (not shown) may be placed in various positions. The user 1 may listen to 5.1 channel stereophonic sound through the speakers 2, 3, 4, 5 and 6, as shown in FIG. 1A, and the sub woofer (not shown). A binaural impulse response is measured when the sound is transferred from each of the speakers 2, 3, 4, 5 and 6 to the user 1.
FIG. 1B is a block diagram schematically showing a stereophonic sound output apparatus that down-mixes a conventional 5.1 channel audio signal to a 2-channel audio signal to be output. In FIG. 1B, an audio signal FL output from the speaker 3 disposed at the front left side, an audio signal FR output from the speaker 4 disposed at the front right side, an audio signal RL output from the speaker 5 disposed at the rear left side, an audio signal RR output from the speaker 6 disposed at the rear right side, and an audio signal C output from the speaker 2 disposed at the center are transmitted to a FL synthesizer 10, a FR synthesizer 20, a RL synthesizer 30, a RR synthesizer 40, and a C synthesizer 50, respectively.
The synthesizers 10, 20, 30, 40, and 50 individually convolute each audio signal with the binaural impulse response measured in FIG. 1A. Adders 70 and 80 mix the audio signals output from each of the synthesizers 10, 20, 30, 40, and 50, and output 2-channel audio signals LEFT and RIGHT. An audio signal SW output from the sub woofer (not shown) is a 0.1-channel audio signal with a low frequency having a wavelength much larger than the size of the head of the user 1. The audio signal SW is mixed and output by the adders 70 and 80 without convolution of the binaural impulse response.
Since ten (10) impulse responses having a length corresponding to the reverberation time of a space are convoluted by the audio signals output respectively through the speakers 2, 3, 4, 5, and 6 as described in connection with FIG. 1A, memory usage and computation times are high. A simplified method is described in Schroeder, M. R., “Natural Sounding Artificial Reverberation”, J. Audio Engineering Society, Vol. 10, No. 3 (1962). Schroeder's reverberation device has a simple structure, and the reverberation is obtained using less computation. However, the frequency characteristics are not smooth, and unnatural sound is output due to a high regularity of reflection time delay.
Additionally, in the case of a reflection generated in a real room, a single reflection enters both ears. However, in the case of headphones, if there is no pair of reflections played back through each channel formed taking an interaural time difference (ITD) between two channels into consideration, a group of unnatural early reflections may be formed differently from the reflection generated in real rooms. This is because, in the case of the headphones, signals played back through each channel do not enter different ear pieces.